Duh. Once I rode my skateboard down the biggest hill in town and I made it to the bottom but was going so fast I couldn't stop and I ran over some gravel at the bottom. I wiped out and tore a huge chunk of flesh from my elbow, about the size of a half dollar and seep enough that I could see the muscle (though I didn't actually damage the muscle, just thinned out the fat layer). No stitches, just some antibiotic ointment and gauze.